Unable to pickle functions that are decorated with `functools.lru_cache()`

## Observed behavior

The cloudpickle module is unable to pickle functions that are cached via the `functools.lru_cache` decorator function.

The following minimal example triggers the issue on Python 3.6.5 and cloudpickle version 0.5.3:
```python
# minimal.py
from functools import lru_cache
import cloudpickle

@lru_cache()
def cached_func():
pass

cloudpickle.dumps(cached_func)
```
Resulting in the following traceback:
```bash
$ python minimal.py
Traceback (most recent call last):
File "minimal.py", line 8, in
cloudpickle.dumps(cached_func)
File "/Users/csadorf/miniconda3/lib/python3.6/site-packages/cloudpickle/cloudpickle.py", line 895, in dumps
cp.dump(obj)
File "/Users/csadorf/miniconda3/lib/python3.6/site-packages/cloudpickle/cloudpickle.py", line 268, in dump
return Pickler.dump(self, obj)
File "/Users/csadorf/miniconda3/lib/python3.6/pickle.py", line 409, in dump
self.save(obj)
File "/Users/csadorf/miniconda3/lib/python3.6/pickle.py", line 507, in save
self.save_global(obj, rv)
File "/Users/csadorf/miniconda3/lib/python3.6/site-packages/cloudpickle/cloudpickle.py", line 642, in save_global
return self.save_dynamic_class(obj)
File "/Users/csadorf/miniconda3/lib/python3.6/site-packages/cloudpickle/cloudpickle.py", line 497, in save_dynamic_class
self.save_reduce(tp, (obj.__name__, obj.__bases__, type_kwargs), obj=obj)
AttributeError: 'functools._lru_cache_wrapper' object has no attribute '__bases__'
```
Versions:
```bash
$ python -V
Python 3.6.5 :: Anaconda, Inc.
$ python -c "import cloudpickle; print(cloudpickle.__version__)"
0.5.3
```

## Expected behavior

The cloudpickle module should handle cached functions by either pickling or ignoring the cache or at least should fail with a descriptive error message.

Start with the minimal.py example from the issue and reproduce the AttributeError using a function decorated with functools.lru_cache(). Trace cloudpickle's handling of the cached function and verify that the expected behavior is covered by a test, including successful pickling or a descriptive failure.

Scritto dal modello di indicizzazione a partire dal testo della issue.